Television transmitting or receiving system



Dec. 1940- E. L. 0. WHITE ETAL I 2,227,050

TELEVISION TRANSMITTING OR RECEIVING SYSTEM Filed Feb. 12, 1958 \I K I JINVENTORS E.L. C. WH/TE BY 0.1..RATSEY A' 'r RNEY Patented Dec. 31, 1940TELEVISION TRANSMITTING R RECEIVING SYSTEM i i Eric Lawrence CaslingWhite, Hillingdon, and 0sbert Linn Ratsey, Havant, England, assignors toElectric & Musical Industries Limited, Hayes, Middlesex, England, acompany of Great Britain Application February 12, 1938, Serial No.190,192

In Great Britain February 18, 1937 4- Claims. (01. 178-71) Thisinvention relates mainly to television transmitting or receiving systemsin which syn-j chronizing signals are employed which have an amplitudeoutside the amplitude of the picture 5 signals, but is also applicableto any system which employs signals comprising two portions, one portionhaving amplitudes greater than a certain datum value and the otherportion having amplitudes less than said datum value.

In television transmitting systems the signals transmitted are composedof picture signalsinterspersed with synchronising signals, thesynchronising signals being usually in the blackerthan-bl-ac direction.For various reasons it may be desirable to adjust the amplitude of thepicture signals without altering the amplitude of,

the synchronising signals. If, for example, the picture signals and thesynchronising signals are fed through a potentiometer, it will be foundthat on adjustment of the potentiometer to decrease the amplitude of thepicture signals,-the amplitude of the synchronising signals willlikewise be reduced.

The necessity for adjusting the amplitude of the picture signalsrelatively to the synchronising signals may arise in the case wheretelevision signals aretransmitted to a receiver from which the receivedsignals after amplification are retransmitted. In this case it may benecessary to adjust the ratio of i the picture signals to thesynchronising. signals before re-transmission, either to correct formisadjustment of the originally transmitted ratio or for attenuation 35of the synchronising signals due to, non-linear amplification in thetransmitter so. that the ratio can be adjusted to that required for themain transmitter.

In addition, it is frequently necessary during television transmissionsto change from one scanning device to another scanning device or fromone transmitter to another transmitter. This changeover must beaccomplished by fading out the signals from one scanner or transmitter45 and then fading in the signals from another scannerror transmitter.It is not feasible to mix two sets of signals in varying proportion, asin sound faders, since the two sets of synchronising signals may not bein phase. In the case where difierent synchronising signals are employedin the scannersor transmitters, the fading-out of one set of signals andthe fading-in of another set of signals cannot, in the normal way, beeffected, since with a reduction in the amplitudeof the picture signalsa reduction in amplitude of the synchronising signals also occurs, andhence,

at a certain instant during. thechangeover the receiver will not be fedwith synchronising signals.

It is therefore the chief object of the present invention to provide animproved transmitting or receiving system whereby adjustment of theamplitude of picture signals or like signals can be effected withoutsubstantially reducing the amplitude of synchronising or like signals.

According to the main feature of the present invention there is provideda circuit arrangement for use with television signals comprising twoportions, i. e. picture signals and synchronising signals, or for usewith similar signals, one

portion having amplitudes greater than a datum means for varying theamplitude of the picture.

or the like signals, and wherein in order to permit the amplitude of thepicture or the like sig nals to be varied in amplitude withoutsubstantially varying the amplitude of the synchronising.

or thelike" signals, the unilaterally conducting device is arranged topermit the synchronising or the like signals to be substantiallyunaffected by said amplitude varying means. v

In one embodiment of the present invention there is. provided a circuitarrangement for we with television or the like signals comprising apotentiometer device in series with a unilaterally conducting device, towhich both portions of the signals are fed, said unilaterally conductingdevice being so arranged as to pass only the picture.

or the like signals through said potentiometer device, the arrangementbeing such that when said potentiometer device is adjusted to. vary theamplitude of the picture or the like signals, the, amplitude of thesynchronising'or the likesi nals remains substantially unafiected.

In one modification of the above embodiment, in order to reduce theamplitude of the picture signals to zero, the residual signal which isdue to the finite resistance :of the unilaterally conducting device isneutralised by combining with it an equal signal in opposite phase.

The neutralising voltage may be derived from 3 a potential dividingmeans shunting the source of picture synchronising or the like signals,and

in a further modification a valve which acts as a so-called cathodefollower, is included in the circuit in such a manner as to allow theresistance of said potential dividing means to be increased.

According to another embodiment of themesent invention there is provideda circuit arrangement for use with television or the like signalscomprising two channels, one channel passing the picture or the likesignals and includingmeans for varying the amplitude of said pictureorthe like signals, and a second channelwhich includes a unilaterallyconducting device which is so arranged as to pass only the synchronisingor the like signals unaffected by said amplitude varying means, theoutputs from the two channels being combined in a suitable manner.

According to a further embodiment of the present invention there isprovided a circuit arrangement for fading from a first source oftelevision I the like signals'of the second source is increased.

- on continued movement of said common control.

In order that the said invention may be clearly understood and readilycarried into effect it will now bemore fully described with reference tothe accompanying drawingin which:

, Fig. 1 illustrates a circuit constructed in ac cordance with one formof the invention, Fig. 2.is a circuit constructed in accordance withanother formof the invention, and

Figs. 3 and 4 illustrate modification of the in vention shown in Fig. 1.I

Withthe circuit shown in Fig. 1, picture-signals 5 in the positivedirection interspersed with synchronising signals in the negativedirection are appliedto. input terminals 3 and 4,, the signals with theD. 0. component present being fed to the input terminals from a lowimpedance source, 0 such as from a circuit which includes a so-calledcathode-follower valve., The terminal 3 is connected through a,potentiometer 5to the anode of a diode 6, whilst thetermina-l t isconnected to the cathode of the diode through a biassing bat- 55 teryland to one output terminal 8, the other output terminal 9 beingconnected to' the adjustable tapping point of the'potentiometer 5. Thecathode of the diodefi is biassed by the battery 1 to a potential equalto the potential of the black 60 level of the signals applied to theterminals 3 and 4 so that the diode is only rendered conducting vwhenpicture signals are applied thereto, whilst the synchronising signalsrender the diode nonconducting. For example if the peaks of the syn- 5chronising pulses are at zero potential, the'biassing potentials willrequire to be positive and the potentiometer 5, the amplitude of thepicture signals can be reduced substantially to zero or increased fromzero upwards, whilst the amplitude of the synchronising signals issubstantially unimpaired. The diodeB biassed in the manner abovedescribedfunctions as discriminator means whereby the amplitude 'of oneset of signals can be varied without substantially affecting theamplitude of anotherset of signals. Instead of em- 1 ploying a diode,any other suitable construction of unidirectional conducting device maybe employed.

With the arrangement described in connection with Fig. 1, it will beappreciated that a picture may be faded out whilst holding the picturein synchronism during such fading process and, at an appropriate time,the scanning device or W transmitter from which the signals are beingfaded out is switched off and another scanning device or transmitterswitched in simultaneously, the picture from the second scanning deviceor transmitter. being faded in by means of an arrangement similar tothat shown in Fig. 1. The arrangement shown alsopermits of adjustment ofthe amplitude of thepicture,signalsrelatively to the synchronisingsignals whilst maintaining the latter signals of substantially constantamplitude.

The arrangement shown in Fig. 1 may, in some circumstances, not provealtogether satisfactory, since itmaybe difilcult to reduce the picturesignals .to less than about 5% of the maximum amplitude without loss of.the higher frequencies.

Fig. 2 of the drawing illustrates an arrangement in which the signalscan be faded as above described, and in which the picture signals can bereduced to zero without substantial attenuation of the higherfrequencies. In this figure separate channels are provided in parallel,one for the picture signals and the other for the synchronising signals,the picture signal channel having means for varying the amplitude of thepicture signals. The picture signals interspersed with synchronisingsignals in the blacker than black direction are applied to inputterminals I I! and H and to the grid of a triode valve 12 which acts asa linear amplifier through a condenser I 3, a leak resistance l4 beingprovided between grid and cathode as shown. The anode of the valve l'2is connected through a potentiometer l5 to the positive terminal'sourceof anode .current, not shown, whilst the tapping point in thepotentiometer is connected through a condenser l6 to the grid of afurther triode valve H which acts as a linear amplifier'and has aleakresistance l8, as shown. The cathodes of the valves I2 and I! areassociated with biassing resistances l9 and'Zll, each shunted by by-passcondensers'Zl and 22. The path through valve i2, potentiometer l5,condenser l6'to the grid of the valve I1 constitutes'the channel for thepicture-signals and a further valve 23 is provided which constitutes anelement of the channel for the synchronising signals. The anode of valvei2 is connected through a condenser 24 to the grid of the valve 23 aleak resistance 25 being provided between grid and cathode as shown,valve 23, condenser 24 and resistance 25 are. arrangedto function in themanner described" in the specification of British Patent No. 419,441, sothat in this manner the valve 23 which operates in non-linear manneronly conducts when synchronising signals are applied to its controlgrid; The anodes of the valves 23 and I! are connected together andthrough a load resistance 26 to the positive terthe negative terminal ofthe associated source of anode current to which the cathodes of thevalves are connected as shown. It will be seen from the arrangementdescribed, that the amplitude of the picture signals can be varied onadjustment of the potentiometer I5, and'since the synchronising signalsare not only fed through the potentiometer rs, but are also fed directlyfrom the anode of the valve I! to the Valve 23, the amplitude of thesynchronising signals cannot be reduced below a predetermined amount byadjustment of the potentiometer l5.

consequently, amplified and added to the signals appearing in the anodecircuit of valve ll, the output from the terminals 2! and 29 willconsist of picture signals and synchronising signals the latter beingvariable but always of greater amplitude than the synchronising signalsfed to the input terminals l0 and H. In order to reduce the amplitude ofthe synchronising signals to the required constant level, the signalsare stabilised with reference to black level by the use of for examplethe means described in the specification of British Patent No. 449,242,and the signal passed through a limiting stage in order to restore thesynchronising signal to the required amplitude.

In this example also it will be seen that the circuit functions todiscriminate by the provision of separate channels between the picturesignals and the synchronising signalsso that the former signals may bevaried in amplitude without sub stantial decrease in amplitude of thesynchronising signals below their original value.

Where the arrangement shown in Fig. 2 is employed for fading out apicture from one source and fading in a picture from another source, twocircuits similar to that shown may be associated with the twotransmitters and the two potentiometers may be arranged to be operatedfrom a common control knob so that on initial movement of the knob thepicture from one transmitter is faded out and at the mid-position of theknob the first transmitter is switched off and the other transmitterswitched on, the finalmovement of the knob fading in picture signalsfrom the second transmitter.

Fig. 3 of the drawing illustrates a further circuit in which theamplitude of the picture signals can be reduced to zero. It will beappreciated that the reason for the inability of the circuit shown inFig. 1 to reduce the picture signals to zero is due to the finiteresistance of the diode 6 when conducting and also by reason of the factthat it may be necessary to insert resistance in the cathode of thediode in order to mask variations of this finite resistance. Fig. 3shows a circuit embodying a diode operating in accordance with Fig. 1and in which the residual signal is neutralised by combination inopposite phase with an equal and opposite signal. As shown, the tappingpoint 30 is connected to the grid of a triode valve 3|, which if desiredmay be biassed in any suitable manner, the anode of which is connectedthrough a load resistance 32 to the positive terminal of a source ofanode current, not shown, whilst the cathode is connected to anadjustable tapping point 33 on a potentiometer 34 connected across theinput terminals 3 and 4, such that the Since the synchronising signalsare fed to the valve 23 and are,

signal amplitude on the cathode of the triode is substantially the same.as the signal amplitude at the anode of the diode, that is to say thelowest signal amplitude applied to the grid. As result the signal outputobtained at the output terminals 35 and 3B is zero when the tappingpoint 30 is in its lowermost position. r

With the arrangement shown in Fig. 3, the resistance of thepotentiometer 36 cannot be made so high as-to constitute a negligibleload on the input, since the portions of the resistance above and belowthe tapping point 33 are in a given ratio depending upon the residualvoltage to be neutralised, and since the lower portion cannot be largeas it constitutes the biasing resistance for the valve 3|.

Fig. 4 illustrates an arrangement having the. propertiesof the circuitshownin Fig. 3, but in which the resistance of the potentiometer 34 canbe considerably increased compared with the resistance of thepotentiometer 34 of Fig. 3. In this case, the voltage at the tappingpoint 33 is. applied to the grid of a triode valve 31, which, ifdesired, may be biased in any suitable manner, the cathode of which isconnected to the cathode of a triode valve 38 to the grid of which thetapping point 30 is connected, the cathodes of'the two valves 31 and 38being connected together and through a cathode resistance 39 tothe'negative terminal of the associated source of anode current, notshown. The resistance 39 maybe returned with advantage to a potentialmore negative than that of the negative terminal of the source,'allowing the resistance 39 to be made larger than would otherwise bepossible. The anodes of the two valves are connected through loadresistances 4i! and 4| to the positive terminal of the source of anodecurrent. The magnitude of the resistance 39 is such that the valve 38 iscorrectly biassed as an amplifier and so that the potential'of thecathode follows substantially potential variations of the grid of thatvalve. The tapping point 33 on the potentiometer 34 is such that, withthe tapping point 30 of the potentiometer 5'in its lowermost position,the voltage applied to the grid of the valve 31 is equal to thatappearing at its cathode and hence there is substantially no change inthe anode current in valve 31, that is to say, there is substantially nopicture signal output. When synchronising pulses are applied to theinput'terminals 3 and 4, the voltage'on the'grid of'the valve 38 fallsby the full magnitude of the synchronising pulses, whereas the voltageof the grid of the valve 3'! falls by only a fraction of this amplitude.

The effect of the common cathode resistance 39 is to providesynchronising pulses in push-pull relation at the anodes of the twovalves. During decrease of the amplitude of the picture signals byadjustment of the tapping point 30 of the potentiometer 5, picturesignal will also be obtained in push-pull at the anodes of the twovalves 3? and 38. The output terminals 35 and 36, therefore, providepicture signals, the amplitude of which can vary down to zero, andsynchronising pulses of substantially constant amplitude in the samesense as the synchronising pulses applied to the input terminals 3 and4.

In the above description reference has been made to synchronisingsignals in the blackerthan-black direction, but it must be understoodthat the invention can also be applied in cases where synchronisingsignals are in the whiterthan-white direction. If, for example, thesynchronising signals are in the positive direction and the picturesignal in the negative direction then the diodes of Figs. 1, 3 and 4 arereversed. Fig. 2 will, however, only work with the synchronising signalsin the negative direction.

We claim:

1. In a television system wherein picture and synchronizing signals arepresent and constitute a single series of signals, the picture signalsbeing present between synchronizing signals and having an amplitudegreater than a predetermined datum value and the synchronizing signalsoccurring at substantially regular intervals and having an amplitudeless than the predetermined datum value, and wherein the picture signalsmay be altered in intensity to the exclusion of and by an amountdifferent from the change in intensity of the synchronizing signals,comprising a unilateral conducting device and an impedance connected inseries, means for impressing the picture and synchronizing signalsacross said device and impedance, means for biasing said device againstconductivity for amplitudes of less than substantially the saidpredetermined datum value, and an output circuit including said deviceand a portion of said impedance, the portion of the impedance includedin the output circuit being less than-the entire impedance.

2. In a television system wherein a single series of signals includingpicture and synchronizing signals are present, the'picture signals beingpresent between synchronizing signals and. having an amplitude greaterthan the predetermined datum value and the synchronizing signalsoccurring at substantially regular intervals and an output circuitconnected to said cathode and an adjustable point along said resistance.

3. In a television system wherein a series of signals are present, aportion of the signals having an amplitude in excess of a predeterminedvalue, .the remaining portion of the signals having an amplitude lessthan a predetermined value, and wherein the portion having an amplitudein excess of the predetermined value may be altered in intensity withrespect to the amplitude of the remaining portion of the signals,comprising a discharge device having a cathode and an anode, aresistance, means for connecting one end of said resistance to saidanode, means for applying said series of signals to said cathode and theother end of said resistance, means for maintaining said cathodepositive with respect to said anode by an amount substantiallyequivalent to said predetermined value, and an output circuit connectedto said cathode and an adjustable point along said resistanceintermediate the ends thereof.

, 4. In a television system wherein a single series of signals includingpicture and synchronizing signals are present, the picture signalshaving an amplitude greater than the predetermined datum value and thesynchronizing signals having an amplitude less than the predetermineddatum value, and wherein the picture signals alone may. be altered inintensity to the exclusion of the synchronizing signals, comprising a'first discharge tube including a cathode and an anode, a firstresistance, means for connecting one end of said resistance to saidanode, a second ,re-, sistance connected in parallel with said firstdischarge tube and said first resistance, a second discharge tubeincluding a cathode, a control electrode and an anode, means forconnecting the cathode of said second discharge tube to a point alongsaid second resistor, means for conmeeting said control electrode'to apoint along said first resistor, means including a load impedance formaintaining the anode of said sec-- ond discharge tube positive withrespect topsaid cathode, means for maintaining the cathode of said-firstdischarge tube normally positive with respect to said anode by an amountsubstantially equivalent to said predetermined datum value,

means for applying said single series of signals across said secondresistance and said seriesconnected first dis-charge tube and firstresistor, and an output circuit coupled to the cathode and anode of saidsecond discharge tube.

ERIC LAWRENCE CASLING WHITE. OSBERT LINN RATSEY.

